1 Field of the Invention
The present invention relates to rotating media storage devices (RMSDs). More particularly, the present invention relates to an RMSD that adaptively estimates a read access time to a second track based on a binned radial position of the second track in order to optimize data transfer rates to and from a media.
2. Description of the Prior Art and Related Information
Computer systems rely on rotating media storage devices (RMSDs), which often employ a moveable head actuator to frequently access large amounts of data stored on the media. One example of an RMSD is a hard disk drive. One critical aspect of designing a RMSD in an extremely competitive market is its performance, typically measured by benchmark programs. The benchmark programs measure, among other things, the time required to perform a series of data transfer commands such as read commands. The time required to execute a given command is dependent on several factors including mechanical latency, cache size, and the efficiency of cache management.
One key aspect of mechanical latency in a RMSD is media rotational speed. In a competitive environment, products such as disk drives at a comparable price point offer comparable rotational speeds. Another aspect of mechanical latency is seek time, which generally measures the time required to move a head actuator from a current position to a target position. The seek time is determined by the mechanical characteristics of the head actuator, sometimes termed head stack assembly, and a seek profile which defines, during the time the actuator is accelerated and decelerated en route to the target position, an instantaneous desired velocity or acceleration/deceleration designed to achieve an efficient and predictable arrival of the head supported by the actuator over the target position on the media. Disk drive makers work diligently to improve the mechanics of the head stack assembly and the seek profile used to manage the seek operation. Nevertheless, disk drives at comparable price points tend to exhibit similar seek time characteristics.
In an effort to overcome limitations imposed by mechanical latency, disk drives have employed cache memory to reduce command execution time by providing host computer requested data from a cache memory, rather than from the disk directly. This technique is useful because oftentimes requested data is data that is repetitive or sequential with previously requested data. The disk drive, having anticipated such requests, is able to furnish data from a semiconductor memory and thus avoids mechanical latency. The process of reading data which is sequential with host requested data may involve reading data which precedes the host-requested data (pre-read) and data which follows the host-requested data (post-read). Utilizing these types of read caching algorithms improves data throughput.
Today, most RMSDs utilize a read access time to allocate the amount of time it should take to seek from a first track and to settle on a second track in order to start reading data from the second track. Typically, these read access times include periods of time for performing post-read operations on the first track and pre-read operations on the second track. Usually these read access times are looked up from a look up table and are based merely on the absolute distance from the first track to the second track and rotational latency.
Unfortunately, sometimes the read access time is not of sufficient length to complete the seek and settle operation to the second track. When this occurs, the RMSD must perform another revolution of the disk to capture the missed data resulting in an entirely wasted revolution of the disk and seriously degrading the data transfer rate of the RMSD. This often occurs when the second track is an outer diameter track. One of the reasons for why his happens is because when the head of the RMSD moves to an outer diameter track of a rotating disk there are sources present there that generate increased Position Error Signal (PES) values (e.g. due to vibration) and as a result high PES values are present. Thus, it takes longer to settle on an outer diameter track. This variable is not taken into account in the read access time. Moreover, oftentimes increased PES values occur at outer diameter tracks of the disk due to imperfections, which were written into the outer diameter tracks during the servo writing process.
Looking at FIG. 1A, an example of a seek and settle operation for an RMSD with a read access time and a standard read caching algorithm is illustrated. Data 10 is read along Track-1 and a seek command to Track-2 (e.g. an inner diameter track) to read data is received at point 12. Consequent to this event, the time to seek to the second track is calculated as the read access time 11. The read caching algorithm typically performs a post-read 14 along Track-1 and then a seek is initiated at point 13. Line 15 illustrates the movable head of the RMSD moving from Track-1 to Track-2. Particularly, as line 15 illustrates, the movable head seeks and settles to Track-2. A read condition is then met when the seek and settle operation is complete and an on-track condition is declared at point 16. The read caching algorithm causes a pre-read 17 on Track-2 and then further proceeds to read the desired data 18. As previously discussed, by performing the post-read 14 and further the pre-read 17, and reading this data into semiconductor memory, the overall data transfer rate can be improved. However, this only holds true as long as the moveable head seeks and settles to the second track within the read access time 11.
On the other hand, as shown FIG. 1B, another example of a seek and settle operation with a read access time and a standard read caching algorithm for an RMSD is illustrated. In this case the moveable head of the RMSD is moving from a first track to an outer diameter track. Here, data 20 is read along Track-1 and a seek command to Track-2 to read data is received at point 22. Consequent to this event, a read access time 11 to seek and settle to the second track is calculated. The read caching algorithm typically performs a post-read 24 along Track-1 and a seek is initiated at point 23 based on the read access time 11. Line 25 illustrates the movable head of the RMSD moving form Track-1 to Track-2. In particular, as line 25 illustrates, due to the inherent sources that generate increased PES values (e.g. due to vibration) at an outer diameter track, the resulting PES values at the outer diameter Track-2 are high, as previously discussed, and the moveable head takes considerably longer to seek and settle to Track-2, as opposed to the seek and settle operation of FIG. 1A to an inner diameter track.
Particularly, as shown in FIG. 1B, the movable head does not settle to the outer diameter Track-2 until point 16 (at which point reading can begin). Thus, the moveable head does not settle on the intended pre-read portion 17. Unfortunately, the pre-read portion 17 and missed data 18 still need to be read and the disk of the RMSD must perform another revolution to capture the missed data. This results in an entirely wasted revolution of the disk and seriously degrades the data transfer rate of the RMSD.